This invention relates to fused silica. More particularly, the invention relates to methods and apparatus used for adding metals such as aluminum to fused silica glass articles.
As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows are typically manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, including manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. These optical members are used in various apparatus employed in environments where they are exposed to high-power ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other high-power ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
In overview, boules are manufactured by reacting silicon-containing gas molecules in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The term xe2x80x9cboulexe2x80x9d is used herein with the understanding that the term xe2x80x9cboulexe2x80x9d includes any silica-containing body formed by a flame deposition process.
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) and larger can be routinely produced in large production furnaces. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The principal optical axis of a lens element made from such a blank will also generally be parallel to the boule""s axis of rotation in the furnace. For ease of reference, this direction will be referred to as the xe2x80x9caxis 1xe2x80x9d or xe2x80x9cuse axisxe2x80x9d. Measurements made in a direction perpendicular to the axis 1 or use axis will be referred to as xe2x80x9coff-axisxe2x80x9d measurements.
As the energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers are exposed to increased levels of laser radiation. Fused silica members have become widely used as the manufacturing material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV and deep UV (DUV) wavelength ranges, which includes lasers operating at about 193 nm and 248 nm wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam due to the fact that each individual photon is of higher energy. In such excimer laser systems, fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optical members.
It is known that laser-induced degradation adversely affects the performance of fused silica optical members by decreasing light transmission levels, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, are susceptible to laser damage to various degrees.
One of the known methods for reducing absorption levels in the glass is to reduce total metal impurity levels of metals such as sodium, aluminum, and iron. For example, one known way of reducing metals impurities in the glass involves treating the refractory materials used in the fused silica production furnace with a halogen gas. Further details on this method are described in U.S. Pat. No. 6,174,509. Another known method of improving the transmission and durability of fused silica optical members is disclosed in U.S. Pat. No. 6,174,830, which discloses annealing silica glass members for 10 or more hours at 1000xc2x0 C. so that the hydrogen content of the member is 5xc3x971018 molecules/cm3 or less. While the method in U.S. Pat. No. 6,174,830 is advantageous in that it produces optical members having excellent properties, the annealing process takes a considerable amount of time and expense to produce such members.
Fused silica members can also exhibit transient absorption. As described in the article xe2x80x9cTransient absorption in excimer-exposed silica,xe2x80x9d by Charlene Smith, Nicholas Borrelli and Roger Araujo, Applied Optics, Vol. 39, No. 31, 5778-5784 (Nov. 1, 2000), the contents of which are incorporated herein by reference, transient absorption can take two forms. In one form, the transmittance of glass in the UV region recovers somewhat when the irradiation source is removed and redarkens quickly when reexposed to light. In the second form, absorption occurs upon the initial irradiation of the glass, and this absorption decreases with constant illumination of the optical member. This type of transient absorption will be referred to herein as an xe2x80x9cabsorption spike.xe2x80x9d This absorption spike is problematic because optical elements contained in optical equipment such as stepper systems must be exposed to a sufficient number of pulses to work through the absorption spike and reduce to the absorption value to avoid the undesirable effects of absorption changes in an optical member when the optical member is placed in service. This exposure process requires optical equipment manufacturers to devote time and resources to work through the absorption spike to reduce absorption to an acceptable level.
The presence or the absence of the absorption spike has been traced to the level of molecular hydrogen dissolved in the glass. Typically, optical members containing high amounts of hydrogen, for example, a concentration of 1019 molecules/cm3, do not have a measurable absorption spike. Accordingly, manufacturing of optical members with high amounts of molecular hydrogen is one way of reducing this absorption spike. Hydrogen can be introduced during the boule formation process or after formation of the boule.
Commonly assigned, copending United States Patent application, entitled xe2x80x9cFused Silica Containing Aluminum,xe2x80x9d naming Dan Sempolinski as inventor, discloses that inclusion of low levels of aluminum ( greater than 50 ppb) in fused silica glass articles improves transmission, and the absorption spike is significantly reduced when compared to fused silica glass articles that do not contain additional aluminum. Low levels of aluminum, for example, as low as about 50 parts per billion and as high as about 900 parts per billion have been observed to improve the aforementioned properties in fused silica optical articles. Normally, impurity levels of aluminum in fused silica glass articles presently manufactured by the assignee of the present invention are typically below 20 parts per billion. In the past, the assignee of the present invention has manufactured fused silica articles that contain around 50 parts per billion of aluminum. However, these fused silica articles also contained high levels of other metal impurities such as sodium and iron (e.g., sodium levels as high as 100 ppb), with the internal transmission of fused silica articles at 193 nm being no greater than about 99.4%/cm.
In the course of experimentation with varying the metal levels in fused silica glass to optimize the properties of fused silica articles, applicants discovered that introduction of metals into fused silica articles at the parts per million and parts per billion levels is challenging and difficult to achieve. Accordingly, a need exists to provide methods and apparatus capable of introducing metals in fused silica glass in low levels, for example, at the parts per million and parts per billion level.
It would be desirable to provide methods and apparatus for manufacturing fused silica glass articles that exhibit improved transmission and a decreased absorption spike. It would be advantageous if the production of such fused silica glass articles could be provided without having to resort to expensive and time consuming treatments such as annealing or prolonged irradiation after formation of the optical members. It would be advantageous to provide a method of adding metals to fused silica glass articles that would allow for the controlled introduction of metals at the parts per million and parts per billion levels.
The invention relates to methods and apparatus for producing fused silica glass articles. As used herein, the term xe2x80x9cfused silica glass articlexe2x80x9d includes boules or bulk pieces of fused silica produced in a furnace, blanks cut from boules, and fused silica optical members manufactured from blanks of fused silica. The production of fused silica optical members may involve finishing steps including, but not limited to cutting, grinding, polishing and/or coating the piece of fused silica glass. Methods and apparatus are provided that are capable of producing fused silica glass articles having high internal transmission and high resistance to optical damage by ultraviolet radiation in the ultraviolet wavelength range.
According to one embodiment of the invention, a method of adding metal to fused silica glass articles is provided. The method includes the steps of producing a silicon-containing gas stream in vapor form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to silica and flowing a second gas stream over a powdered, organometallic precursor and sublimating the precursor to enrich the second gas with the organometallic precursor and to provide a dopant-containing gas stream. As used herein, the terms xe2x80x9cdopant,xe2x80x9d xe2x80x9cdopedxe2x80x9d and doping refer to the addition or incorporation of metals or other materials to fused silica. According to one embodiment of the invention, such doping includes the addition of metals to the fused silica during the flame deposition process. In one embodiment, the method further includes the steps of mixing the silicon-containing gas stream and the second gas stream, flowing the mixed gas streams into the flame of a combustion burner to form amorphous particles of doped fused silica, depositing the amorphous particles onto a support and consolidating the deposit of amorphous particles into a transparent glass body.
In another embodiment of the invention, the second gas stream is heated to a first temperature, the precursor is heated to a second temperature to sublimate the precursor, and the temperature of the saturated, dopant-containing gas is heated and maintained at a third temperature. According to another embodiment, the precursor includes an organometallic chelate. In another embodiment, the precursor includes a nonhydrolyzable organometallic chelate. In another embodiment, the dopant level in the fused silica member is controlled below 100 parts per million, preferably below 900 parts per billion, and typically in the range of 50 ppb and 900 ppb. According to another embodiment of the invention, the organometallic chelate includes aluminum. In still another embodiment, the organometallic chelate includes aluminum acetylacetonate. For this embodiment, the first temperature is between about 100xc2x0 C. to 160xc2x0 C., the second temperature is between 0xc2x0 C. and 10xc2x0 C. below the first temperature, and the third temperature is greater than 175xc2x0 C.
In still another embodiment, the consolidated glass body is cooled from a temperature greater than 1800xc2x0 C. to a temperature less than 1200xc2x0 C. over a time period greater than four hours. Applicants have found that by adding aluminum to fused silica glass and slowly cooling the fused silica glass results in a glass articles that have high internal transmission, for example greater than 99.5%/cm at 193 nm and a decreased absorption change.
Certain embodiments of the invention relate to a method of manufacturing a fused silica optical member including the step of providing a gas stream containing a silicon-containing compound in vapor form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to silica. This embodiment further includes the steps of adding a metal to the gas stream in a manner such that the amount of the metal in the fused silica optical member can be controlled to a level of at least 50 parts per billion and flowing the gas stream into the flame of a combustion burner to form amorphous particles of doped fused silica. In another embodiment, the step of adding the metal to the gas stream includes sublimating a metal containing precursor, such as, for example an organometallic chelate. In one embodiment, the precursor includes aluminum acetylacetonate.
Still another embodiment of the invention relates to an apparatus for doping metals into a fused silica glass article. According to this embodiment, the apparatus includes a heated precursor chamber for holding a powdered metallic precursor and a heated stream of carrier gas in fluid communication with the precursor chamber, wherein the carrier gas is passed through the precursor chamber to provide a mixture of carrier gas. In this embodiment, the apparatus further includes a delivery line for delivering the mixture of carrier gas and precursor to a gas stream containing a silicon-containing compound in vapor form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to silica. In another embodiment, the precursor chamber includes a screen for containing the precursor and for allowing gas to pass through the precursor chamber. In another embodiment, the apparatus is capable of controlling the amount of metal introduced into the fused silica at a level of 50 parts per billion. In still another embodiment, a plurality of combustion burners in fluid communication with the silicon containing gas stream are provided to provide a flame for combusting the silicon-containing gas and converting the gas to silica particles.
According to the present invention, methods and apparatus are provided that enable the controlled introduction of metallic dopants into fused silica at levels as low as 50 parts per billion. The methods and apparatus of the present invention enable the production of fused silica articles exhibiting lower absorption and increased internal transmission.
Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.